In a lithium ion secondary battery or another secondary battery, an electrode, an electrolyte, or the like is accommodated in a battery case. In such secondary batteries, internal pressure rises to an abnormal level when hydrogen gas is produced in the interior of the battery case due to a defect. Therefore, a safety valve is provided such that a breakage groove is formed in the battery case, and when the internal pressure of the battery case rises, the breakage groove is cleaved by the internal pressure and the internal pressure is released (see Patent Document 1).
In the battery case disclosed in Patent Document 1, a recessed part having a trapezoidal cross section and an ellipsoidal planar shape is formed in the bottom surface of the battery case, and a breakage groove is formed annularly along the side surface of this recessed part. With this configuration, the internal pressure at which the breakage groove cleaves can be controlled via the wall thickness (remaining thickness) in the bottom part of the breakage groove.